In the production of glass a generalized process is followed wherein glass forming materials such as sand, soda, lime, feldspar, dolomite and recycled glass (commonly referred to as cullet) are mixed into a batch which is melted and fined in a furnace operating at atmospheric pressure and at temperatures of about 800-1300.degree. C. and of about 1300-1500.degree. C., respectively. The glass material is then cooled for conditioning, forming and annealing. (See Tooley, The Handbook of Glass Manufacture, 3d. Ed.)
During the melting phase, gases such as CO.sub.2 and N.sub.2 are formed due to various well known reactions. These gases form bubbles or imperfections in the melt which must be eliminated. Fining is the physical and chemical process by which these gases are removed from the glassmelt. As part of this process, various materials known as fining agents are added to the batch glass prior to mixing. The primary role of these agents is to release gases in the glassmelt at proper fining temperatures which then diffuse into gas bubbles in the glassmelt. As the bubbles become larger their relative buoyancy increases and they rise to the surface of the glassmelt where the gases are released. According to Stokes' law, the speed at which the. bubbles move through the glassmelt may be increased by reducing the viscosity of the glassmelt. By increasing glassmelt temperature, more fining gases are released and the viscosity of the glassmelt is reduced. This is why the fining process takes place in the hottest zone in the furnace.
Since a conventional glass melting furnace operates at atmospheric pressure above the glassmelt, the fining process of gas bubbles inside the glassmelt takes place at about 1 to 2 atmospheres depending on the depth of the glassmelt as well as any additional pressure caused by the surface tension effect on small bubbles. (Beerkens, R. G. C., Glastechnische Berichte Vol. 63, 1990, pp. 222-236). Rapid release of fining gases can take place when the glassmelt temperature is increased, the fining agent is dissociated and the partial pressure of fining gases exceeds the internal pressure of gas bubbles. Thus, an effective fining agent for atmospheric pressure glass melting and fining processes must have a property such that as the temperature of the glassmelt is increased to the temperature range where the viscosity of the glassmelt is sufficiently low, i.e., 1300 to 1500 C. for soda lime glass, a large amount of fining gases is released. Gases such as O.sub.2, N.sub.2, CO.sub.2, H.sub.2 O and argon, whose solubilities in glassmelt depend only weakly with temperature, have not been considered as effective fining agents, either alone or in combination with other known fining agents in an atmospheric pressure glass melting and fining process.
An example of a common fining agent is sodium sulfate, which dissociates to form SO.sub.2 and O.sub.2 gases according to the following reaction: EQU SO.sub.4.sup.2- (in melt)=SO.sub.2 (gas)+1/2O.sub.2 (gas)+O.sup.2- (in melt )
Other sulfate compounds include calcium sulfate and barium sulfate, as well as sulfate containing materials such as filter dust and slags are also used in the batch materials to provide sulfate in glass.
The amount of sulfate used in glass batch depends on the type of glass melted. Typical ranges of sodium sulfate used per metric ton of glass product are 6 to 12 kg (3.4 to 6.7 kg as SO.sub.3) for float and oxidized plate glass, 5 to 8 kg (2.8 to 4.5 kg as SO.sub.3) for flint bottle glass, 4 to 7 kg (2.2 to 3.9 kg as SO.sub.3) for green bottle glass, and 5 to 10 kg for textile fiber glass (E-glass). When a large fraction of the charge materials consists of cullet, the requirement for sodium sulfate may be reduced below the ranges shown above since cullet already contains sulfate.
About half of sulfate in the glass batch may be retained in the glass product and the other half evolves as SO.sub.2 gas during fining and batch melting. SO.sub.2 gas evolves during batch melting by reacting with carbon and other compounds, including reducing gases in the furnace atmosphere, if present.
SO.sub.2 as well as other toxic and particulate emissions from glass furnaces are of serious environmental concern. One possible solution to this problem is the use of oxy-fuel combustion, which uses commercial grade oxygen in place of air. While oxy-fuel combustion has been demonstrated to reduce NO.sub.x emissions from gas furnaces by 80 to 99%, methods to reduce other toxic and particulate emissions are still being sought. A major source of these emissions is fining agent reaction products such as SO.sub.2 which are released during the melting and fining processes.
It is also known that gaseous SO.sub.2 plays a role in the formation of particulate emissions in the following manner. NaOH which has formed at the glassmelt surface, by the reaction of water vapor and sodium oxide in glassmelt, reacts with SO.sub.2 and O.sub.2 in the regenerator and flue duct to form Na.sub.2 SO.sub.4 as well as other sulfate compounds. These compounds condense to form sub-micron sized particles.
There are currently three primary methods used to reduce SO.sub.2 emissions: 1) reduction in the amount of sulfate in a glass batch, 2) controlling the burner firing conditions and atmosphere within the furnace to reduce the loss of sulfate during batch melting, and 3) installation of an SO.sub.2 scrubber in order to clean flue gas.
For most commercial glass furnaces the amount of sulfate in the glass batch has been adjusted to a lowest acceptable level to operate the furnace properly and to achieve good glass quality. So a further reduction in sulfate would result in poor glass quality. For example the "theoretical minimum limit of sulfate requirement" for float glass is defined as the amount of sulfate retained in glass plus 0.05 wt. % as SO.sub.3 evolved at the fining zone (W. R. Gibbs and W. Turner, "Sulfate Utilization in Float Glass Production", 54th Conference in Glass Problems, The Ohio State University, November 1994). Therefore, assuming 0.25 wt. % SO.sub.3 retention in glass, the minimum sulfate requirement is equivalent to 5.3 kg of sodium sulfate per metric ton of float glass. The actual amount of sulfate mixed in the batch materials is typically much greater.
It is known that impinging flames and reducing combustion atmospheres tend to accelerate batch sulfate reactions and result in premature release of SO.sub.2 in the batch melting zone. Thus, an adjustment of the burner firing conditions and the furnace atmosphere over the batch area may reduce sulfate emissions without adversely affecting the glass quality.
For glass furnaces equipped with a bag house or an electrostatic precipitator, greater particulates generation may not create a problem. For these furnaces, greater volatilization of sodium in the furnace by high velocity burners or by higher operating temperatures may reduce SO.sub.2 vapor emissions by forming more Na.sub.2 SO.sub.4 particulates. This is not a preferred option, however, as higher sodium volatilization could create refractory corrosion problems. Likewise, installation of an SO.sub.2 scrubber is not preferred as this incurs additional costs. Thus, the most preferred option to decrease SO.sub.2 emissions in atmospheric pressure glass melting and fining processes is to reduce batch sulfate, provided that glass fining is not adversely effected.
In the vacuum refining method of glass (Kunkle et al., U.S. Pat. No. 4,738,938 and Pecoraro et al., U.S. Pat. No. 4,919,700) molten glass is transferred in a refining vessel and removal of gas bubbles is accelerated by reducing the pressure in the vessel. Contrary to an atmospheric pressure glass melting and fining process it is preferable to create a sufficiently large volume of foam for effective removal of dissolved gases. In such a system it is not necessary to use a common fining agent such as sodium sulfate that releases gases with increasing glassmelt temperature.
In FIG. 1, the solubility of SO.sub.3 and H.sub.2 O are plotted against pressure, assuming gas solubility of 0.1 and 0.108 wt. % respectively, at 1500 C. The solubility SO.sub.3 is known to decrease proportionally with the partial pressure of SO.sub.3, while the solubility of H.sub.2 O is known to decrease proportionally to the square root of the partial pressure of H.sub.2 O. For example, if the glassmelt contains 0.4 wt. % SO.sub.3 and the pressure is reduced to about 0.4 atm, SO.sub.3 will start to evolve. Similarly, if the glassmelt contains 0.04 wt. % H.sub.2 O and the pressure is reduced to about 0.14 atm, H.sub.2 O will start to evolve. All dissolved gases will start to come out of the glassmelt as the pressure of the glassmelt is reduced below the saturation points of the dissolved gases. Thus, in a vacuum process, any gas with sufficient solubility in glass can be used to expand gas bubbles and create foam for refining, and a fining agent such as SO.sub.3 is not required.
The role of fining agents and dissolved gases in an atmospheric process, which relies upon an increase in glassmelt temperature for refining, is known to be fundamentally different. In FIG. 2, the solubility of SO.sub.3 and H.sub.2 O are plotted against temperature at atmospheric pressure. As the temperature of the glassmelt increases, the solubility of SO.sub.3 decreases sharply and reaches about 0.3 wt. % at 1400 C. for float glass. If this glass initially contains 0.3 wt % of SO.sub.3 (which exists as SO.sub.4.sup.2- in the glassmelt), then SO.sub.3 would start to evolve at about 1400 C. as the temperature of the melt is increased. (See Gibbs and Turner cited above). By comparison, the solubility of H.sub.2 O is insensitive to temperature, or even increases slightly with temperature according to some data (F. Kramer, in "Gas Bubbles in Glass", International Commission on Glass, 1985, p. 105, Table II, Ref 10!.)
Thus, even if glass is saturated with water at low temperature, H.sub.2 O would not be expected to evolve as the temperature of glass is increased. As such, gases such as O.sub.2 and H.sub.2 O whose solubilities in glassmelt are substantially lower than SO.sub.3 and further, whose solubilities depend only weakly with temperature, have not been considered as effective fining agents in an atmospheric pressure glass melting and fining process.
Another significant difference between vacuum and atmospheric glassmaking processes is that in an atmospheric process the formation of foam in the fining zone must be minimized as it reduces the heat transfer from the flames and furnace crown to the glassmelt and as such reduces the glassmelt temperature required for fining. Although the vacuum refining process can substantially eliminate the need for conventional fining agents, the high costs of such a system make it uneconomic to use in the commercial glass making processes except for the manufacture of a few special glasses.
Accordingly, it is an object of this invention to provide an atmospheric pressure glass melting and refining process which allows for the reduction in batch sulfate as well as other known fining agents required without adversely effecting the quality of the glass produced.
It is known that water acts as an effective fluxing agent in glassmaking operations by forming hydroxyl groups in the glass molecular structure. A number of methods to increase the quantity of hydroxyl groups in glass have been tried. For example, steam or moist air has been bubbled through molten glass in an electrically heated glass melting furnace (E. N. Boulos et al, in "Water in Glass: A Review", J. Canadian Ceramic Soc. Volume 41, 1972); heating with hydrogen based combustion has been carried out, either above the glass surface or by submerged combustion (K. J. Won et al, in U.S. Pat. No. 4,545,800); and alkali hydroxyl compounds such as sodium hydroxide, potassium hydroxide and lithium hydroxide have been added to the glass batch during melting (Doi et al in "Uniform Introduction of OH Group into Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 Glass By Addition of LiOH.H.sub.2 O", Japan J. Appl. Phys. Vol 12, 1973). Finally, under oxy-fuel firing, the concentration of water dissolved as OH groups in glass becomes 30% higher as compared to air combustion (Kobayashi and Brown "Is Your Glass Full of Water?" 56th Ann. Conf. on Glass Problems at the University of Illinois (Urbana-Champaign) October, 1995).
However, it has not heretofore been recognized or disclosed that there is a relationship between water content and the atmospheric pressure fining process such that one may effectively reduce the amount of conventional fining agent required to remove a given amount of undissolved gases from a glassmelt.